Question

I want to read a long number from file then increment it and write it back to file.
I am struggling with the convertion from string to long and back again.

I tried:

double id = atof("12345678901"); //using atof because numbers are too big for atio()
id++;
ostringstream strs;
strs << static_cast<long>((static_cast<double>(threadId)));
string output = strcpy_s(config->m_threadId, 20, strs.str().c_str());

But that converts the input to a negative or wrong number.

Answer 1

atoi is for normal integers. There’s also atol and atoll (_atoi64 in windows):

//long long id = atoll( "12345678901" );
long long id = _atoi64("12345678901"); // for Visual Studio 2010
id++;
// write back to file here

As suggested by one commenter, use strtoll instead of the ato* functions:

char * data = "12345678901";
long long id = strtoull( data, NULL, 10 );
id++;

Since you’re using C++ here, you should just pull it straight from the fstreams:

long long id;
{  
   std::ifstream in( "numberfile.txt" );
   in >> id;
}
id++;
{
   std::ofstream out( "numberfile.txt" );
   out << id;
}